Polycarbonates are well-known resins which have good property profiles, particularly with respect to impact resistance, electrical properties, optical clarity, dimensional stability and the like. Polycarbonates are used in applications requiring extreme toughness, transparency, resistance to burning, and maintenance of useful engineering properties over a wide temperature range. Typical applications include: bubble helmets for astronauts, canopies for supersonic aircraft, furnishings for commercial aircraft, break-resistant windows, transparent bullet-resistant laminates, impact-resistant lenses, combination electrical insulation and mechanical housings for appliances, automotive instrument panels, and the like.
One polycarbonate resin that has been used successfully in such environments is based on bisphenol A ("BPA"). The relevant art prior to BPA-polycarbonates and the subsequent development of this material as an engineering resin is reviewed in H. Schnell, Chemistry and Physics of Polycarbonates, Wiley-Interscience, New York (1964) and K. Johnson, Polycarbonates Recent Developments, Noyes Beta Corp., Parkridge, N.J. (1970).
While BPA-polycarbonate provides significant benefits, this resin and related amorphous polycarbonates have certain disadvantages. Most notably, the resistance of amorphous polycarbonates to organic solvents is rather limited The tendency of organic solvents to crystallize, craze, crack or mar the surface of objects made from such polycarbonates naturally limits their application. For example, tensile strains may readily be "frozen in" to polycarbonate parts during fabrication by injection molding and, when compounded with stresses encountered in service, result later in crazing on exposure to unfavorable environments. The presence of solvents or solvent vapors in such environments may alter threshold conditions for crazing to the point where parts under mechanical stress will fail. Therefore, environmental conditions as well as stresses to be encountered in fabrication and in service must be considered carefully in designing for polycarbonates.
There have been a number of attempts to improve the solvent resistance of BPA-polycarbonate resins. One effort involved the use of a cross-linking agent, as disclosed, for example, in U.S. Pat. Nos. 4,604,434, 4,636,559, 4,701,538, and 4,767,840. This technique is often effective, but difficulties are sometimes encountered by reason of swelling of the cross-linked polycarbonate in the presence of organic liquids, and loss of ductility with increasing levels of cross-linking agents.
Another limitation of certain BPA-polycarbonates is flame resistance. Although polycarbonate resins will normally merit the rating of "self-extinguishing" according to ASTM Method D 635, they will often receive poorer ratings under UL-94 testing procedures.
Thus, there continues to be a need for BPA-polycarbonates that have increased solvent and flame resistance in combination with high impact strength and conventional processability.